1. Field of Invention
The invention pertains to the art of molding plastics, and in a more particular sense is specifically directed to a method of molding a bowling pin.
2. Description of the Prior Art
The prior art is known to include a number of patents disclosing plastic bowling pins and methods of making the same.
The state of the prior art as exemplified by such patents, however, does not suggest, in any bowling pin intended for use in full size bowling lanes in place of the wooden pins now used, a solid, fully molded, plastic bowling pin the entire body of which is formed in a single piece, is not of foamed plastic (though it may be if desired), is not machined, and yet has all the characteristics of wooden pins in respect to weight, strength, sound on impact, center of gravity, and resistance to dents, marks, and other damage.
A method for providing a bowling pin having this complete particular collection of properties has heretofore not been devised, so far as is known, in the prior art. Patents such as those listed above include, in some instances, a requirement for full or at least partial machining of the bowling pin during the manufacture thereof. In other instances, the body of the bowling pin is of multipart construction, requiring special interfitting of pieces, and again, in some instances, a machining operation. In still other instances, it has been suggested that foamed plastic be used, with special shells being externally provided, for the purpose of approximating the properties of a conventional, wooden bowling pin.
It has further been suggested that a molded plastic bowling pin be made according to a method utilizing a thermoplastic resin, in particular polypropylene, to which is added a blowing agent, that is to say, a foaming agent to an extent to produce a cellular structure. In this process, the molding procedure is entirely an injection molding procedure without an accompanying compression molding step. There is no suggestion, in the particular prior art development, for combining, in a method for making a bowling pin, the steps of subjecting a high density polyethylene first to injection and then to compression molding, with the compression to be effected by advancement of a core pin specially shaped to exert the compressive forces against the material in a manner particularly productive of uniform density, the minimizing of surface and internal irregularities, and the production of a finished product closely approximating a wooden pin in playing characteristics and durability.
None of the prior art pins have been conceived, designed, and constructed in such manner as to truly solve all the problems involved in duplicating the appearance, weight, dimensional stability "click" and scoring capabilities of a conventional wooden pin.